This invention relates to a method for increasing the signal-to-noise ratio in non-destructive testing of a sample by a phase-stepped optical inspection system which, particularly, but not exclusively, is suitable for use with optical shearography employing a shearing interferometer.
A particular type of phase-stepped optical inspection system for non-destructive testing or evaluation of a sample is an optical shearography system using a shearing interferometer.
Such contemporary shearography systems measure the surface form of an object in a static position and after it has been subjected to a stressing force. Usually, the stress is administered by a pressure reduction chamber or by thermal loading. The two images that characterize the unstressed and stressed states are subsequently subtracted to yield an image that contours the stress-induced distortion. If the stressing force has been applied effectively, then sub-surface defects in the test object, such as disbonding between the skin and core materials of a composite panel, may be visualized in this image.
Unfortunately, there are several factors that limit the effectiveness of shearography for non-destructive testing (NDT) namely:
1. Shearography is based on speckle interferometry, which is inherently noisy. PA1 2. If the stressing force is applied gradually over an extended period, then the initial and final images may become de-correlated, due to environmental instability, and no result is produced. PA1 3. The dynamic range of shearography is restricted, because of the limited spatial resolution of video based image acquisition. PA1 4. Transient features, such as air currents, can mask the presence of defects. PA1 correlated speckle images successively stepped in phase are generated and captured of a sample to be tested or evaluated, by illuminating the sample, while in a static unstressed state, with coherent radiation, PA1 the sample is illuminated with coherent radiation, stressed incrementally at predetermined stress increments, and correlated speckle images successively stepped in phase are generated of the incrementally stressed sample at the predetermined stress increments and captured, PA1 the phase-stepped speckle images of the unstressed sample and the phase-stepped speckle images of the incrementally stressed sample are used to calculate the phase before and after stressing and differenced to extract the incremental phase change, PA1 the magnitude and sign of each incremental phase change is inspected, and if the phase difference between successive measurements increases by more than .pi., 2 .pi. subtracted from the measurement values, or if the phase difference between successive measurements decreases by more than .pi., 2 .pi. is added to the measurement value and PA1 the phase at each point in each speckle image is calculated with improved accuracy by summing the nearest neighbor phase differences in the image that are weighted by the square of their respective modulations, where the modulation is a measure of intensity variation with phase variation and the result of this calculation is normalized by dividing by the sum of the modulations.
A conventional technique known as phase stepping helps reduce image noise by eliminating stationary intensity patterns and enhancing image contrast. However, even if phase stepping is employed, the results are often too noisy and not sufficiently repeatable to be dependable for non-destructive testing or evaluation of safety critical structures.
There is thus a need for an improved method whereby the noise of shearography images is reduced and the repeatability of results is improved to an extent where shearography becomes viable for production non-destructive testing.